The present invention relates to circuitry for supplying a signal for driving an induction motor. More specifically, the invention relates to circuitry for controlling the slip frequency of the motor.
An induction motor is always powered by an alternating current drive signal. The rotational frequency (i.e. speed) of the motor is roughly proportional to the frequency of the alternating current drive signal. When the motor is accelerating, its rotational frequency is less than the frequency of the drive signal. The difference between these two frequencies is defined as the slip frequency.
It is well known that by controlling the slip frequency, the power consumption and output torque of a motor can be controlled. The prior art, in some instances, has made control of the slip frequency manually adjustable. The most common approach, however, is to regulate the slip frequency electronically, to maintain a predetermined constant value. Usually, this is accomplished by providing circuitry to sense the motor speed and maintain a fixed slip frequency drive signal in relation thereto.
A prior art slip frequency control is exemplified in U.S. Pat. No. 4,286,203 to Robert J. Ehret, assigned to Beckman Instruments, Inc., the assignee of the present invention. Other relevant prior art includes U.S. Pat. Nos. 4,042,862 to Franklin; 3,983,463 to Nabae et al.; 3,753,064 to Agarwal et al.; 3,731,169 to Burgholte et al. and 3,568,022 to Domann et al.
A disadvantage of the constant slip frequency systems is that the optimum slip frequency for obtaining a desired motor characteristic, such as for example, maximum torque is a function of the motor's rotational frequency. In particular, at low motor speeds, such as when a motor is accelerating from rest, the slip frequency provided by a constant slip frequency system is usually too high. As a result, the torque of the motor is reduced and the time required to accelerate to the desired operating speed is prolonged.
In one of the aforementioned references, U.S. Pat. No. 3,983,463 to Nabae et al., a system is disclosed wherein motor slip frequency is controlled as a function of the current drawn by the motor. While this technique may offer some advancement over the prior practice, it has the disadvantage of being overly complex and therefore impractical.